Question: Which of these numbers is prime? ${35,\ 47,\ 55,\ 60,\ 87}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 35 are 1, 5, 7, and 35. The factors of 47 are 1 and 47. The factors of 55 are 1, 5, 11, and 55. The factors of 60 are 1, 2, 3, 4, 5, 6, 10, 12, 15, 20, 30, and 60. The factors of 87 are 1, 3, 29, and 87. Thus, 47 is a prime number.